In recent years, long-term storage stability has become an important property required for an image formed by using an ink-jet recording system.
Widely recognized causes of deteriorating image quality of an ink-jet image in long-term storage include exposure of the image to sunlight and to illuminating lamps or the like. In order to improve the long-term storage stability of the ink-jet image, intensive studies have been made for improving lightfastness of the image. Lightfastness can be evaluated using a color difference (ΔE), and an optical density difference (ΔOD) in an L*a*b* calorimetric system (colorimetric system standardized by Commission Internationale de I'Eclairage (CIE) in 1976, which is defined in JIS Z8729 in the Japanese Industrial Standards (JIS)) between before and after exposure tests to light (see U.S. Pat. Nos. 6,702,882 and 6,866,380).
However, even when the color difference (ΔE) values are the same, the extent of deterioration of ink-jet images appear to be different to human eyes, depending on respective hue ranges of yellow, magenta and cyan. This is because the human eyes are highly sensitive to a change of hue angles. That is, when the hue angles are changed, the human eyes perceive a change of colors. Further, since the optical density difference (ΔOD) is not an indicator of a change of color tones like the hue angle, the sensitivity of the human eyes against the optical density difference (ΔOD) is lower than that against a value of hue angle change (Δh). Since an ink set for ink-jet recording defined by the color difference (ΔE) or the density difference (ΔOD) does not correspond to an evaluation made by the human eyes, neither satisfactorily shows long-term storage stability and thus long-term storage stability has not been adequately defined.
The optical density (OD value) of each of the three primary colors (yellow, magenta and cyan) and black can be measured. When the optical density (OD value) of a medium color (for example, flesh color) represented by a subtractive color mixing method using the three primary colors is measured, the value obtained does not faithfully reflect the color density of the medium color itself (showing as respective optical densities of colors decomposed to yellow, magenta and cyan, or showing as the optical density of the color changed into mono-color).